Over the past several years, the quality of color images produced by computer printers such as inkjet printers and color laser printers has improved to the point where such images now rival those achievable through color photography. In addition, many such printers are capable of printing on a wide range of media, including paper stocks of various qualities, sizes and thicknesses. When an optical scanner is used in combination with a printer and the appropriate print media, original documents can be scanned and printed to produce copies which can be difficult to distinguish from the original. The scanner can be a separate peripheral device, or can be combined with the printer in a single “all in one” peripheral device, such as the OfficeJet™ Pro Series product family from Hewlett-Packard Company, the assignee of the present application. “All in one” devices, which may be connected to a computer or may be used as a stand-alone color copier without requiring an attached computer, commonly include an ink jet based printing system that has a removable printer cartridge. Others have laser printing based systems. An optical scanning system is also provided. A user can scan a document with the scanning system and then print the document with the printing system. Image information can also be transmitted to and received from an attached computer. In some instances, a modem is provided to allow scanned text and other images to be transmitted via facsimile over telephone lines to a remote location.
At the same time as the color image quality obtainable from scanner-printer configurations has improved, the cost of these devices has decreased making them more widely available and more readily accessible. This increased copy quality and device accessibility has raised concerns about the proliferation of illegal counterfeiting of documents such as paper currency, negotiable instruments, identification papers, and the like. A related concern is the undesired copying of proprietary or confidential business documents intended for only limited distribution.
Two general classes of solutions to these problems have been developed. The first type, “active” counterfeit prevention systems, attempt to recognize during the scanning process documents which should not be copied, and if such a copy-prohibited document is detected, prevent the printing of a copy. Such systems require advance knowledge of the image characteristics of the types of documents that are not to be copied, and must reliably detect these documents when copying of them is attempted. As a result, they tend to be expensive, in part because accurate determination of prohibited versus acceptable originals is difficult, and also because the system must be programmed to recognize each type of document intended to be protected.
The second general class of systems are “passive” marking systems. Such systems tend to be less complex in part because they do not attempt to prevent the copying of certain documents, but rather imprint all copies they produce with additional forensic information. Frequently this forensic information will identify the manufacturer, model number, serial number, or owner of the device, thus allowing the source of a particular printed copy to be traced to a particular system or person. However, since it is likely that most printed copies produced by the system will be of perfectly legitimate documents, it is highly desirable that the forensic information be virtually unnoticeable to the unaided human eye, in order to avoid undesirably degrading the print quality of the copy. As a result, the size, pattern, location, repetition, and color of the forensic markings must be carefully designed so as to ensure that the markings are imprinted in as visually unobtrusive a manner as possible. At the same time, the forensic markings must also be readily detectable and accurately interpretable so as to identify the device on which the copy was printed to be determined from the printed copy with near-perfect accuracy, should the genuineness of a document be called into question and thus its origin determined.
Some previous passive marking systems print small forensic markings in blank areas of the image. However, when the print density of a page is sufficiently high, such as when a photographic image is printed, there may not be enough remaining blank space to accommodate sufficient forensic markings to ensure accurate recognition. Other previous passive marking systems print forensic markings in a color, frequently yellow, which is not readily perceived by the human eye. As a result, these systems do not necessarily require blank image space for the forensic markings, but rather they may print the yellow markings in regions of the media that also contain the image. However, since the image itself may already contain yellow image content, forensic markings overlaid in these regions may be corrupted to such a degree that they misidentify the copying device, unless a mechanism exists to validate the markings.
Consequently, there is still a need for a passive marking system which minimizes the impact of forensic markings on print quality by forensically marking the printed medium with a small amount of imperceptible information overlaid on the image content, while ensuring that the passive marking system that produced the printed medium can accurately be identified from the forensic markings.